We present new methods for creating color or grayscale images that are printed with classical inks such as cyan, magenta, yellow and a white diffusing ink on a metallic substrate. A first goal is to optimize the surface coverages of the classical cyan, magenta, yellow inks and of the white ink in order to create color prints on a metallic surface that look bright and colorful under specular reflection and also look good under non-specular observation conditions. The second goal is to provide new means for the prevention of counterfeits. Two main effects are available. A first effect enables viewing on the same metallic print one grayscale image in specular viewing mode and a second independent grayscale image in non-specular viewing mode. The second effect enables hiding a pattern such as text, graphics or a grayscale image within the printed color image in one viewing mode, specular or non-specular and showing that pattern in the second viewing mode, non-specular or specular, respectively.
These varying color images enable preventing counterfeits of security documents and valuable articles and at the same time create surprising effects that can be used to increase their attractiveness and aesthetics. Therefore, the invented color imaging methods may also be useful for decoration, advertisement, art and amusement parks.
In respect to the prior art, U.S. Pat. No. 7,491,424 B2, Reproduction of security documents and color images with metallic inks, filed 19 May, 2003, to inventors R. D. Hersch, F. Collaud, and P. Emmel, teaches a security feature obtained by combining a metallic ink and standard inks printed on paper to create color images with embedded patterns. In contrast to the present work, the prints were obtained with a metallic ink and classical inks printed on white paper and the patterns were only hidden when viewed under non-specular observation angles. In addition, since the metallic ink particles are not completely flat, the specularity of the metallic print is less pronounced than with the present invention, where the print substrate is formed by a very flat metallic layer possibly coated with a transparent non-absorbing ink attracting layer on which the colored and white ink halftones are printed.
Pjanic and Hersch (also inventors in the present invention) created a color reproduction workflow for printing with classical inks on a metallic substrate. In order to establish the correspondence between amounts of inks and resulting color under specular reflection, they used an ink spreading enhanced cellular Yule-Nielsen spectral prediction model. This model is calibrated with patch reflectances measured under specular reflection. The resulting printed colors appear bright and colorful under specular reflection, but look dark and colorless under non-specular reflection, see publication by P. Pjanic and R. D. Hersch, Specular color imaging on a metallic substrate, In Proc. IS&T 21st Color Imaging Conference, 61-68 (2013). The authors did not consider the possibility of using a white diffusing ink to create appealing color images also in non-specular viewing mode.